Comparison between the use of Co3O4 or CoO on microstructure and electrical properties in a varistor system based on SnO2

This comparative study pursues to clarify the differences between the addition of CoO or Co3O4 as a dopant on the non-linear electrical behavior and microstructure characteristics of a varistor system based on SnO2. The samples were fabricated according to the conventional ceramic processing using the nominal composition: (99.99-X)%SnO2 – 0.05%Cr2O3 – 0.05%Sb2O5 – X%CoO/Co3O4 where X = 0, 1, 3 and 5 mol%. The TGA-DTA analysis revealed essential differences in the thermal stability of CoO and Co3O4 during the sintering process, being CoO the oxide that induced the most visible changes. SEM micrographs, supported by EDS and XRD, showed remarkable grain growth even with a slight addition of cobalt oxide in comparison with the reference sample. The use of CoO holds the average grain size (6.7 μm), composition (SnO2), and shape homogeneity. In the case of Co3O4, a decreasing tendency in the average grain size and an increase in the residual porosity due to the in-situ formation of Co2SnO4 phase is observed. The electrical analysis revealed similarities on the alpha value for all molar percentages of both cobalt oxides: ∼9.6 and ∼8.1 units for CoO and Co3O4, respectively. In contrast, the breakdown electrical field shows notable differences: 1.08–1.16 kV cm−1 for CoO and 1.08–2.46 kV cm−1 for Co3O4. The evidence of this study confirms previous findings and suggests the existence of an essential role for the phase transitions of cobalt oxide on the final performance of SnO2-based varistors.